your supply gauge wire is completely dependent on your power requirements. Since the fuse block you're looking at is a 12 circuit, and rated to 30amp per circuit, and if you're running all 12 AT 30 amp you'd be drawing 4,320 watts. An automotive rated 10ga wire less than 3ft will easily be able to dissipate enough heat without overheating due to voltage loss. If you're worried about it you can oversize the wire to 8ga, but it'd be overkill, especially since you won't be running all circuits at 30amp.